A base bias circuit operating like a constant voltage source is indispensable for a power amplifier using a common-emitter bipolar transistor. The constant voltage source is more suitable than a constant current source as the bias circuit from the following reasons.
It is assumed that a RF input is applied to the common-emitter bipolar transistor given a bias to a base under the constant voltage source. In case where input RF power is sufficiently low, the common-emitter bipolar transistor operates with a low signal. Therefore, a collector current is equal to a collector bias current flowing in a such state that no signal is given to the amplifier.
On the contrary, as the input RF power is increased, the collector current of the common-emitter bipolar transistor is increased so as to reach several times or higher of the collector bias current. Due to the increase of the collector current, a higher saturation output and lower distortion can be realized.
In the meantime, in the case where the bias is given to the base under the constant current source, the collector current is constantly kept to hFE times of the base bias current, so that the collector current is not increased even if the input RF power is increased.
Accordingly, when the collector bias current is set in the same manner that the base bias is given under the constant voltage source, a gain compression upon a high-signal operation occurs under a lower input RF power. This degrades saturation characteristic, reduces additional power efficiency and deteriorates linearity.
Further, when the collector bias current is equal to the collector current under such a case that the base bias is given under the constant voltage source and the input RF power is high, a high collector current flows even when no RF signal is given or the input RF power is low. Therefore, consumption power is problematically increased.
From the aforementioned reasons, the base bias circuit operating like the constant voltage source is indispensable for the power amplifier using the common-emitter bipolar transistor. Specifically, an output resistance in a direct current state of the base bias circuit may be equal to or lower than a base input resistance in the direct current state of the common-emitter bipolar transistor of the amplifier.
For the base bias circuit, the following facts will be required.
At first, the bias voltage given to the base must be strictly specified because the common-emitter bipolar transistor has an extremely high mutual conductance gm.
Therefore, it is necessary that the base bias circuit has such a structure that a generated base bias voltage is not affected by fluctuation of a power supply voltage.
Moreover, the collector current of the common-emitter bipolar transistor is exponential function of temperature. Therefore, the base bias circuit must be constituted so that the generated base bias voltage is varied in dependence upon fluctuation of environment temperature to keep the collector bias current constant.
In a CDMA system portable phone, a transmission power must be controlled. In the base bias circuit, it is therefore required that the generated base bias voltage is functionally variable in accordance with a control signal from the external in order to reduce the consumption power of a power amplifier under a low transmission power.
FIG. 2 shows an example of a conventional base bias circuit.
A base bias circuit 52 is composed of an npn type bipolar transistors 53, 54, 55 and a resistor 56. All of the bipolar transistors 54, 55, 53 and 58 are manufactured by the same semiconductor process, and a ratio of emitter areas is set 1:n:1:n. A collector of the bipolar transistor 58 is directly connected to an RF load in a high-frequency state while it is directly connected to a power supply in a DC state.
Initially, consideration will be made of such a case that the RF input power is not given. In the circuit illustrated in FIG. 2, a portion consisting of the bipolar transistors 54, 55 and another portion consisting of the bipolar transistors 53, 58 are symmetrically constituted in the DC state except that the collector of the bipolar transistor 55 is connected to the base of the bipolar transistor 54 while the collector of the bipolar transistor 58 is directly connected to the power supply in the DC state.
Herein, if the voltage VCE between the emitter and the collector is 0.3-0.5 or higher, the portion consisting of the bipolar transistors 54, 55 and the portion consisting of the bipolar transistors 53, 58 are considered to be symmetrically and completely constituted in the DC state because the collector current of the bipolar transistor is not almost dependent upon VCE in general. Specifically, an emitter area ratio of the bipolar transistors 54, 55 and an emitter area ration of the bipolar transistors 53, 58 are 1:n, respectively.
Further, the base potential of the bipolar transistor 54 is kept equal to the base potential of the bipolar transistor 53. From the above-mentioned facts, the base-emitter voltage VBE of the bipolar transistor 55 is substantially equal to VBE of the bipolar transistor 58, and VBE of the bipolar transistor 54 is substantially equal to VBE of the bipolar transistor 53.
Therefore, the collector currents flowing along the bipolar transistors 55 and 58 are equal to each other while the collector currents flowing along the bipolar transistors 54 and 53 are also equal to each other. In this event, the bipolar transistor 53 placed in an output portion of the base bias circuit 52 forms an emitter follower. Taking this into consideration, it is found out that the output voltage of the bias circuit 52 is kept to a value lower with VBE than the base potential of the bipolar transistor 53. In other words, the base bias circuit operates like the constant voltage source.
In case where this fact is analyzed in more detail, the output resistance of the bias circuit 52, namely, the direct current resistance viewed the side of the bias circuit 52 from the emitter of the bipolar transistor 53 is substantially equal to a direct current base resistance of the power transistor 58.
In this event, it is assumed that hFE of the bipolar transistor is sufficiently high, and the base current of the bipolar transistor 54 and the base current of the bipolar transistor 53 are negligibly low for the collector current of the bipolar transistor 55.
The base potential of the bipolar transistor 54 is substantially equal to 2×VBE. In the case where a voltage given to a control terminal is set to Vref and a resistance value of the resistor 56 is set to R, the current flowing along the resistor 56 is given by (Vref-2×VBE)/R. This becomes the collector current of the bipolar transistor 55 in almost such a state.
As described above, the collector currents flowing along the bipolar transistors 55 and 58 are equal to each other. Taking this into consideration, the collector current of the bipolar transistor 58 is also equal to (Vref-2×VBE)/R. Therefore, it is found out that the collector current is not affected by the power source voltage.
Herein, VBE of the bipolar transistor is generally almost constant irrespective of the collector current. Taking this into account, it is confirmed that the collector current of the bipolar transistor 58 is controlled as linear function.
In this event, the bias voltage given to the base of the bipolar transistor is directly controlled. Taking this into consideration, the collector bias current becomes the exponential function, and therefore, slight fluctuation of the base bias voltage causes large fluctuation of the collector bias current.
In contrast, the collector bias current becomes the linear function of the control voltage in the base bias circuit 52, and therefore, the fluctuation of the collector bias current for the fluctuation of the control voltage is suppressed to a sufficiently low value.
Further, when the environment temperature is varied, the characteristic of each bipolar transistor is also varied. As mentioned above, in the circuit illustrated in FIG. 2, the portion consisting of the bipolar transistors 54, 55 and the portion consisting of the bipolar transistors 53 and the power transistor 58 are symmetrically constituted. To this end, the affect of the characteristic fluctuation of the bipolar transistor 54 and the affect of the characteristic fluctuation of the bipolar transistor 53 are canceled to each other while the affects of the characteristic fluctuations of the bipolar transistor 55 and the power transistor 58 are canceled to each other. As a result, such a structure is not readily subjected to the affect of the environment temperature.
However, the conventional technique illustrated in FIG. 2 has several problems.
As a first problem, the emitter area of the bipolar transistor 55 must be equal to the emitter size of the power transistor 58. Even if the power transistor, for example, has an output of about 1 W for use in the portable phone, a chip size is extremely large.
Therefore, the bias circuit comprising the bipolar transistor 58 having the same emitter as the power transistor 58 has the chip size larger than the power transistor 58.
As a second problem, the collector current flowing along the bipolar transistor 55 is equal to the collector bias current flowing along the power transistor 58. This means that the consumption current of the bias supply circuit 52 is substantially equal to the consumption current of the power transistor 58.
As a third problem, a choke inductor 62 is necessary between the bias supply circuit 52 and the power transistor 58 in order to prevent the RF signal from leaking to the bias circuit.
The output portion of the bias circuit 52 is structured by the emitter follower consisting of the bipolar transistor 53. Herein, the base potential of the bipolar transistor 53 is constantly kept to about 2×VBE by an operation of a circuit block. Taking this into account, it is found out that the output impedance of the emitter follower consisting of the bipolar transistor 53 is relatively low under high-frequency.
Accordingly, in order to prevent the input RF signal to the power transistor 58 from leaking to the side of the bias circuit, the choke inductor 62 is required. In the case where an exterior part is used as the choke inductor 62, a mounting cost and a cost for the exterior part is additionally necessary. In case where an inductor device formed on a semiconductor substrate is used as the choke inductor 62, the chip area is increased.
As the conventional technique for solving the first problem and the second problem, the ratio of the emitter areas of the bipolar transistors 54, 55, 53, 58 is set to 1:n:m:m×n in the bias circuit illustrated in FIG. 2.
In this case, the emitter area ratio of the bipolar transistors 54 and 55 is set to 1:n, the emitter area ratio of the bipolar transistors 53 and 58 is also set to 1:n, and the circuit block consisting of the bipolar transistors 54, 55 and the circuit block consisting of the bipolar transistors 53, 58 keeps symmetrical characteristic.
Therefore, functions required for the base bias circuit can be realized. Namely, the bias voltage given to the base is not affected by the fluctuation of the power supply voltage, the base bias voltage is varied in dependence upon the fluctuation of the environment temperature so as to keep the collector bias current constant, and the generated base bias voltage is variable in accordance with the control signal from the external.
Further, the collector current flowing along the bipolar transistor 55 becomes low with 1/m times of the collector current flowing along the power transistor 58, and therefore, the reduction of the consumption power can be realized. Moreover, the emitter area of the transistor having the largest emitter size becomes low with 1/m times, and therefore, the circuit area is reduced in accordance with a method of selecting m.
However, such a third problem that the choke inductor is necessary is not solved. Although the circuit area becomes lowest in the case of m=n, two transistor each having the emitter area of n is required, and there is a predetermined limit for reducing the area of the bias circuit.
Moreover, although the consumption current of the bias circuit becomes lowest in the case of n=1, the bipolar transistor having the same emitter area as the power transistor is necessary in the bias circuit, so that the circuit area is not reduced.
Thus, the conventional circuit has the limit for reducing the consumption current and area. Further, the choke inductor for preventing the leak of the RF power is indispensable between the base bias circuit and the power transistor, the exterior part and the additional mounting cost is necessary, or an on-chip inductor having a large area is required.
It is therefore an object of this invention to provide a base bias supply circuit which has a small chip area and a low consumption current without a choke inductor between a base of a power transistor and a bias supply circuit and which is for use in a bipolar transistor, and an amplifier circuit using the same.
Disclosure of this Invention
A base bias circuit according to this invention supplies a bias current to a base of a common-emitter bipolar transistor of an npn type for a power amplifier.
The base bias circuit comprises first through third bipolar transistors of an npn type and first and second resistors integrated on a semiconductor substrate and a base bias current control terminal.
The first resistor is inserted between the base bias current control terminal and the first bipolar transistor.
The second resistor is inserted between a base of the second bipolar transistor and the first bipolar transistor.
A base of the second bipolar transistor is connected to a collector of the third bipolar transistor.
An emitter of the second bipolar transistor is connected to a base of the third bipolar transistor.
An emitter of the third bipolar transistor is grounded. A collector of the first bipolar transistor is connected to a positive power supply. An emitter of the first bipolar transistor is directly connected to the base of the bipolar transistor for the power amplifier.
In this event, a third resistor may be inserted between a connection point of the first and second resistor and a base of the first bipolar transistor, and the emitter of the third bipolar transistor may be grounded via a fourth resistor.
Further, it may be short-circuited by the use of a metal wiring instead of the second resistor.
In addition, the first and second bipolar transistors may be replaced by n-type MOSFETs, and a buffer circuit may be inserted between the first resistor and the base bias current control terminal.
For example, the buffer circuit comprises a fourth bipolar transistor of an npn type and fifth and sixth resistors. An emitter of the fourth bipolar transistor is grounded and the base thereof is connected to the base bias current control terminal via the fifth resistor.
A collector of the fourth bipolar transistor is connected to the first resistor and the sixth resistor, and the sixth resistor is connected to a power supply terminal.
It is assumed that a resistance of a circuit consisting the first through third resistors and the second and third bipolar transistors, viewed from the base of the first bipolar transistor is defied as R, a mutual conductance of the first bipolar transistor is defined as gm and a common-emitter current amplification factor is defines as h21.
Under such a circumstance, an emitter area of the first bipolar transistor and resistances of the first through third resistors are selected such that an impedance given by (1/gm+R/(h21+1)) is higher than an impedance of the base of bipolar transistor for the power amplifier in a predetermined frequency band and is equivalent to an impedance of the base of the bipolar transistor for the power amplifier in a direct current.
The first through fourth bipolar transistors may be hetero junction bipolar transistors formed on an chemical substrate, and the first through fourth bipolar transistors may be Si homo BJTs formed on a Si substrate.
The first through fourth bipolar transistors may be SiGe hetero junction bipolar transistors formed on a Si substrate.
Further, the bipolar transistor for the power amplifier is formed on the same semiconductor substrate as the base bias circuit, and the base bias circuit and the bipolar transistor for the power amplifier are connected by a metal wiring formed by a semiconductor production process to constitute the power amplifier.